function TreeNode(val){
    this.val = val
    this.left = null
    this.right = null
}

const node = new TreeNode(1)  //{val: 1, left: null, right: null}
const node2 = new TreeNode(2)  //{val: 2, left: null, right: null}
const node3 = new TreeNode(3)  //{val: 3, left: null, right: null}
node.left = node2
node.right = node3
